1. Field of the Invention
The present invention relates to a rigid cover for a needle cannula. More specifically, the present invention relates to a rigid cover for protecting and sealing a needle cannula of a prefilled syringe assembly, and methods of manufacturing the cover.
2. Description of the Prior Art It is common practice in the preparation of medications for use by medical workers, to prepackage the medications directly within a sealed hypodermic syringe assembly when such is a necessary vehicle for administration of the medication to a patient. According to the practice, a hypodermic syringe is prefilled with a predetermined amount of medication, and the medication is sealed into the syringe by sealing the distal end of the needle cannula. The syringe is then packaged singly or in a predetermined quantity, into packaging which can be sterilized to maintain the sterility of the syringes until the medication is to be administered to a patient.
In following this procedure, it has often been the practice to seal the distal end of the needle cannula with a resilient needle sheath. A sheath of this type is usually formed of a resilient rubber material with an elongated cavity therein which can accept the needle cannula. Generally, the sheath is formed so as to be capable of forming a friction fit with the hub of the needle cannula to hold the sheath in position over the needle. The cavity is sized so as to either fit snugly around the distal tip of the needle cannula, or allow the distal tip of the needle cannula to pierce the distal end of the sheath and become embedded therein. In either case, the sheath effectively seals the tip of the needle cannula to prevent fluid from flowing therethrough out of the syringe during storage or prior to use.
Although the resilient needle sheath is generally quite successful in sealing the needle cannula, there remain serious concerns in the use thereof. Specifically, due to the pierceability of the sheath, there exists the possibility of medical workers accidentally receiving a needle stick while using the syringe. This usually occurs either during preparation of the syringe for administration of medication to a patient, or during disposal procedures therefor. For example, since the sheath is formed of a resilient rubber, a minimal force (improperly applied) is all that is needed to cause the needle tip to penetrate, and even pierce completely through, the sheath. Therefore, inadvertent pressure on the tip of the sheath while the needle is properly placed therein, may cause the needle to pierce entirely through the sheath's distal end and become exposed.
Further, medical workers often attempt to reinsert the needle into the sheath after the syringe has been used and prior to its disposal, even though this procedure is generally not recommended. Misalignment of the needle with the sheath during reinsertion can cause the tip of the needle to pierce through the side of the sheath wall and contact the hand of the medical worker holding it. Needle protrusion of this nature can readily occur, since the tip of commonly used needles is ground off center (i.e., on a biases) and therefore does not proceed in a linear path when piercing through the resilient sheath material. Instead, the needle tends to follow the off centered point thereof as it passes through the sheath and in effect "skives off" in an arcing path of travel as it passes through the sheath material. If a medical worker is not extremely careful to let the needle center itself in the sheath cavity during reinsertion, the needle may inadvertently contact the side of the sheath cavity and pierce through the sheath material in a surprisingly unexpected direction, catching the medical worker off guard and likely causing a stick to the hand of the medical worker which is being used to hold the sheath.
Although rigid, substantially impenetrable covers are commonly used to cover needle cannula's of hypodermic syringe assemblies, they are generally unsuitable for use on syringes which have been prefilled with a fluid such as a liquid medicament. This is due to their inability to seal the end of the needle cannula from fluid flow therethrough during storage and prior to use.
There have been attempts in the past to develop a substantially impenetrable cover which can seal the end of a needle cannula commonly used on prefilled syringes, while at the same time provide added protection to a medical worker from accidental needle sticks. An example of a needle cover generally describing the background of the present invention is shown in U.S. Pat. No. 4,735,311 to Lowe et al.
Lowe et al. discloses a needle shield which includes an elongated tubular housing having a central bore which varies in cross-sectional diameter along its entire longitudinal length so as to form a first chamber in the proximal part of the bore which can function to guide the needle into a central position in the shield, and a second chamber in the distal portion of the bore adapted to receive a resilient piercable material. The second chamber is of a diameter slightly larger than the diameter of the resilient piercable insert material, and most importantly, includes a tapered wall at an inner end thereof which forms a diameter less than that of the remaining portion of the chamber. The tapered wall functions to tightly squeeze the insert material into generally hermetic sealing relationship therewith when the insert is force thereagainst.
The Lowe et al. device is assembled by inserting the insert material into the second chamber and then applying a precompression force thereto to cause it to seal against the tapered wall. Heat and pressure are applied to the distal end of the cover to force it to partially collapse over the insert and retain it in its proper position.
Although this invention is a substantial improvement over the use of a resilient sheath alone in preventing accidental needle sticks, it nevertheless contains several drawbacks. Specifically, the Lowe et al. shield requires a substantial amount of material to form the shield itself, due to the incorporation of excessive material inside the bore to form the needle guide surfaces and the proximal and distal chambers.
Further, the tapered portion of the distal chamber is very small, due to the relatively small size of the shield as a whole, and is therefore difficult to accurately manufacture on a mass production basis. Since, the design of the Lowe et al. device relies heavily on the uniformly shaped resilient inserts which must properly seal against the tapered section of the distal chamber in order to create the hermetic seal.
Finally, the method of manufacture of the Lowe et al. device requires a precompression of the insert in order to insure proper seating thereof against the tapered section of the chamber, and requires the compression to be held against the insert until the insert is permanently affixed in place by the deformation of the distal end of the shield. This is an added inconvenience in the manufacturing process.
There exists a need in the art to develop a cover for a needle cannula of a prefilled syringe assembly which can seal the needle against fluid flow therethrough and which also is simple to manufacture and relatively inexpensive.
There further exists a need in the art to develop a needle cover which employs the minimum of material and which does not necessarily include design characteristics which require exacting manufacturing standards in order produce a properly functioning device.